Various information recording techniques have been developed following the increase in volume of information processing in recent years. Particularly, the areal recording density of a HDD using the magnetic recording technique has been increasing at an annual rate of about 100%. Recently, the information recording capacity exceeding 200 GB has been required per 2.5-inch magnetic recording medium adapted for use in a HDD or the like. In order to satisfy such a requirement, it is necessary to realize an information recording density exceeding 400 Gbits/inch2.
In order to achieve the high recording density in a magnetic recording medium for use in a HDD or the like, the perpendicular magnetic recording system has been proposed in recent years. In a perpendicular magnetic recording medium for use in the perpendicular magnetic recording system, the easy magnetization axis of a magnetic recording layer is adjusted so as to be oriented in a direction perpendicular to the surface of a substrate. As compared with the conventional in-plane magnetic recording system, the perpendicular magnetic recording system can suppress a so-called thermal fluctuation phenomenon in which the thermal stability of a recorded signal is degraded due to superparamagnetism so that the recorded signal is lost, and thus is suitable for increasing the recording density.
As the perpendicular magnetic recording medium for use in the perpendicular magnetic recording system, a CoCrPt—SiO2 perpendicular magnetic recording medium (see T. Oikawa et. al., IEEE Trans. Magn, vol. 38, 1976-1978 (2002)) is proposed because it exhibits high thermal stability and excellent recording characteristics. In this CoCrPt—SiO2 perpendicular magnetic recording medium, a magnetic recording layer has a granular structure in which nonmagnetic grain boundaries where SiO2 is segregated are formed between magnetic grains in the form of continuously grown columnar Co crystals with a hcp (hexagonal closest packed) structure, thereby achieving both miniaturization of the magnetic grains and an improvement in coercive force Hc. It is known that an oxide is used for forming nonmagnetic grain boundaries (nonmagnetic portions between magnetic grains), and it is proposed to use, for example, one of SiO2, Cr2O3, TiO, TiO2, and Ta2O5 (JP-A-2006-024346).
Following the increase in recording density of the magnetic recording medium described above, a magnetic head has also shifted from a thin film head to a magnetoresistive head (MR head) and to a giant magnetoresistive head (GMR head), wherein the flying height of the magnetic head from a substrate (magnetic recording medium) has been reduced from about 20 (nm) to about 5 (nm). By reducing the flying height (magnetic spacing) of the magnetic head from the magnetic recording medium in this manner, it has been possible to improve the spacing loss to increase the SNR (signal noise ratio) and thus to achieve a further increase in recording density.
However, while it has become possible to achieve the further increase in recording density by the magnetoresistive head or the giant magnetoresistive head, this type of magnetic head mounted with a magnetoresistive effect element has a problem of causing, as its inherent failure, a head crash or a thermal asperity failure.
The head crash is a failure in which a magnetic head is physically damaged due to its collision with a projection on the surface of a magnetic recording medium. The thermal asperity failure is a failure in which a read error occurs due to heating of a magnetoresistive effect element caused by adiabatic compression of air or contact between a magnetic head and the surface of a magnetic recording medium when the magnetic head passes a minute convex or concave shape on the surface of the magnetic recording medium while flying over it. Since either of the failures is caused by the surface state, i.e. the surface roughness, of the magnetic recording medium, the surface of the magnetic recording medium is required to have extremely high-level smoothness and flatness with respect to a magnetic head mounted with a magnetoresistive effect element.